Another Mirror Mod Issue
1 - About the mirror: you have to apply the Rotation of your object (crtl A > Apply Rotation)

2 - About the mesh of your shoe: you have too much useless edge loops, come back to an earlier version and don't apply the Subsurface and Solidify modifiers.

If impossible, Decimate won't give a good topology, so you can either:


Simplify with the method described in this video (ring selection, checker deselect, select edge loops, delete): https://www.youtube.com/watch?v=Yptda_yKQIU
Or rebuild a new shoe with retopology (create a new object and use snap to stick the vertices to the original shoe)